1. Field of the Invention
The present invention relates to an epoxy resin composition, and a prepreg and a printed wiring board using the epoxy resin composition, and more particularly to an epoxy resin composition including a composite curing agent, and a prepreg impregnated with such a composition and a printed circuit board (PCB) manufactured by using such a prepreg.
2. The Prior Arts
The demand for downsizing the printed circuit boards on which electronic components are mounted is increasingly rising. Accordingly, it is required that the wire width is reduced, the diameter of the through-hole is reduced, and the plating thickness is reduced. However, the reduction of plating thickness can cause the plating to crack or blister when a heat shock is applied to the plating. Thus, the printed circuit boards are required to be highly heat-resistant. In addition, the printed circuit board desirably has a low dielectric constant and a low dissipation so as to be advantageous in increasing the signal transmission speed.
The printed wiring boards are typically manufactured by using the prepregs. For manufacturing a prepreg, in general, a substrate was impregnated with a varnish prepared by dissolving a thermosetting resin, such as epoxy resin, in a solvent, followed by partial curing of the resin, and such impregnated substrate is commonly referred to as prepreg. For manufacturing a printed wiring board, in general, it involves laminating a particular number of layers of the prepregs, and placing a metal foil additionally on at least one outermost layer, and forming a particular circuit pattern on the surface of the metal-clad laminate by etching the metal foil thereon.
The printed circuit board must meet the criteria for heat resistance, dielectric properties, and chemical resistance, and it is worthy of note that the thermosetting resin plays a key role in improving the properties of the printed circuit board. The thermosetting resin can be cross-linked with a multi-functional amine to form an interpenetrating polymeric network (IPN). Although IPNs have favorable properties such as high glass transition temperature and high chemical resistance, they have the drawback of being too brittle to be processed as prepregs. For instance, it proves impossible to cut up such prepregs without a portion of the resin blowing about in the form of a large quantity of dry dust.
TW patent No. 293831 and WO2006004118A1 disclose an epoxy resin composition including an amino-triazine-novolac (ATN) resin used as curing agent for epoxy resin. The amino-triazine-novolac resin can be represented by the general formula given below:

The amino-triazine-novolac resin was prepared by reacting a phenolic compound, an aldehyde compound and a guanamine compound in the presence of an acid catalyst such as oxalic acid or p-toluene sulfonic acid. The phenolic compound used includes, for example, phenol, resorcin, and alkyl phenols such as cresol and xylenol. The aldehyde compound used includes, for example, formaldehyde. The guanamine compound can be represented by the general formula given below:
where R is amino, phenyl, or alkyl such as methyl.
The guanamine compound used includes, for example, melamine, benzoguanamine, and methyl guanamine.
In TW patent No. 293831 and WO2006004118A1, when the amino-triazine-novolac was used as curing agent for epoxy resin, it could improve the heat resistance of the copper-clad laminate made from such epoxy resin. Also, the amino-triazine-novolac resin, which is a phenolic-aldehyde resin, can react well with the epoxy resin used for the fabrication of the copper-clad laminate. However, the copper-clad laminate using amino-triazine-novolac resin as curing agent is not easily to be processed, and has unsatisfactory electrical performance.
Therefore, the various combinations of curing agents have been proposed in order to solve the problems for using a single curing agent. TW patent No. 583258 disclosed a thermosetting resin composition that included benzoxazine and an amino-triazine-novolac resin used as a composite curing agent so that the prepreg manufactured by using such a thermosetting resin composition could have low water absorption and low dielectric properties, but the prepreg manufactured had some disadvantages like low glass transition temperature, and poor heat resistance, and also the printed circuit board manufactured from such prepregs has poor chemical resistance.
On the other hand, diaminodiphenylsulfone (DDS) was conventionally used as an epoxy resin curing agent. The molecular structure of 4,4′-diaminodiphenylsulfone is shown as following:

The drawback to the epoxy resin composition including diaminodiphenylsulfone is that the epoxy curing time becomes longer, and also the peeling strength between the copper foil and the circuit board laminate becomes lower. However, the diaminodiphenylsulfone was still often used in the epoxy resin composition because it can increase the glass transition temperature.
Therefore, there exists a need for providing a composite curing agent that will improve the processibility and the dielectric properties of the epoxy resin used for the fabrication of the copper-clad laminate or PCB while rendering the epoxy resin to have high glass transition temperature, high heat resistance, high peeling strength of the copper foil, and low water absorption.